Abstract:
Efficient use of the scarce radio spectrum is important for accommodating the rapid
growth of wireless communications. This thesis investigates the feasibility of Cognitive
Radio (CR) to exploit unoccupied frequencies for an enhanced spectral utilisation. CR
refers to a wireless architecture that enables dynamic spectrum access, where unlicensed
devices are allowed to operate in temporally/spatially unused licensed channels. Extensive
spectral opportunities have been identified from an intensive measurement study, indicating
that a significant gain in spectral utilisation may be brought by a CR technology.
The main challenge for CR is a robust protection mechanism to guarantee an adequate
Licensed Users (LU) system performance at all times. This requires a reliable spectrum
sensing technique to accurately identify frequency opportunities; and an autonomous
transmit power control algorithm that remains effective in severe fading environments.
Cooperation among CR devices to assist the LU identification process is shown to be an
imperative CR attribute for maximising the CR performance and the overall spectral utilisation.
Cooperation is particularly important for a CR coexisting with a small-coverage
LU system, in which an accurate LU detection mechanism is the predominant limiting
factor of CR system performance. The consequences of insufficient CR collaborations are
an expensive individual CR detector and a large CR-LU separation requirement, yielding
a suboptimal gain in spectral utilisation. The effectiveness of cooperative detection largely
depends on system application, channel characteristics, and CR detector sensitivity.
The feasibility of CR is evaluated via a simulated CR model using the achievable CR
system performance and the corresponding operational requirements as performance metrics.
The fundamental limit of permissible CR transmit power has been derived from an
interference constraint imposed on the CR operation. Assuming the LU accepts a minor
performance degradation (an outage probability < 5%), it has been shown that a low
CR system outage (< 5%) may be achieved while simultaneously providing a reliable LU
identification rate (> 99%) with the modest detector specifications commercially available.
Above all the CR feasibility is strongly subject to the signal characteristics and the
heterogeneous propagation conditions in the CR deployment scenario. An algorithmic
approach is developed which demonstrates that a practical CR system independent of LU
cooperation may be feasible.